1. Field of the Invention
This invention relates to a power discontinuity sensor in an electrically heatable transparency and in particular to a sensor useable with a heatable windshield having a double feed electrical input configuration to a pair of bus bars that will interrupt electrical power to the bus bars when a break is sensed in the bus bar feeds of the windshield that will cause arcing and additional damage to the heatable windshield.
2A. Technical Considerations
It has been known to pass electric current through a bus bar and transparent conductive coating to another bus bar on the surface of a transparency to raise its temperature. A double feed heated transparency, and in particular a windshield, includes first and second bus bars each positioned along opposite edges of the windshield to be heated. Power is provided to at least one of the bus bars by dual leads, each attached to opposite ends of the bus bar. In this fashion, if one of the leads fails, the other can still supply power to the bus bar and maintain the windshield's operation.
One mode of failure of heatable transparencies is overheating of the bus bars which results from arcing when current moves through a defective bus bar. As a result of the arcing, there is a localized area of high temperature that will overheat the windshield. In the instance where the transparency is a laminated windshield, the overheating caused by arcing can damage the plastic interlayer or in severe cases damage the laminated transparency thus adversely affecting its occupant retention capability.
In heatable windshield configurations having single power leads to each bus bar, it has been suggested to detect power discontinuities in the bus bar system by using a sensor lead that either extends along the single power lead and contacts the bus bar system at the juncture between the power lead and bus bar, or a sensor lead that extends along both the bus bar and single power lead and contacts the bus bar system at the free end of the bus bar.
It would be advantageous to have the capability of detecting defects in the bus bars and/or leads of a double feed heatable windshield which may result in overheating and interrupting current flow to the bus bars before the windshield is permanently damaged while maintaining the windshield's ability to continue operation if only one of the two power feed leads is broken.
2B. Patents of Interest
U.S. Pat. No. 3,892,947 to Strengholt teaches an electrically heated panel with anti-shock conductive strips. The electric current conductive strip extends around the periphery of the panel before contacting any portion of an electrically conductive coating on the panel surface. A single break or fracture in the strip will prevent any current from flowing through the coating.
U.S. Pat. Nos. 4,057,671 to Shoop, 4,323,726 to Criss et al., and 4,543,466 and 4,668,270 to Ramus teach a heated laminated window with an electroconductive coating or a wire grid extending between a pair of spaced bus bars. Current passes from one bus bar, through the coating or wire grid, to the opposite bus bar to heat the window.
U.S. Pat. No. 4,565,919 to Bitter et al. teaches a crack detector for an electrically conductive windshield. The circuit includes a sensor arrangement for monitoring the resistance of the conductive film of the windshield and interrupting power applied to the film when the resistance assumes a value indicative of element cracking.